This invention relates generally to semiconductor technology and more particularly to a method of forming polycrystalline silicon within an amorphous silicon film.
Polycrystalline silicon thin film transistors (TFTs) can be used in a variety of microelectronics applications, especially active matrix liquid crystal displays (LCDs).
Thin film transistors (TFTs) used in liquid crystal displays (LCDs) or flat panel displays of the active matrix display type are fabricated on silicon films deposited on a transparent substrate. The most widely used substrate is glass. Amorphous silicon is readily deposited on glass. Amorphous silicon limits the quality of TFTs that can be formed. If driver circuits and other components are to be formed on the display panel, as well as switches associated with each pixel, crystalline silicon is preferred.
Silicon is often referred to as either amorphous or crystalline, including single crystal silicon. The term crystalline silicon can refer to either single crystal silicon, polycrystalline silicon, or in some cases materials with significant quantities of micro-crystal structures. For many application, single crystal material is most desirable. But, single crystal silicon is not readily producible. Amorphous silicon can be crystallized to form crystalline silicon by solid-phase crystallization. Solid-phase crystallization is carried out by high temperature annealing. But, glass substrates cannot withstand the temperatures necessary to melt and crystallize silicon. Quartz substrates can withstand high temperature annealing, but quartz substrates are too expensive for most LCD applications.
Because glass deforms when exposed to temperatures above 600xc2x0 C., low-temperature crystallization (preferably below 550xc2x0 C.) is used for solid-phase processing of silicon on glass. The low-temperature process requires long anneal times (at least several hours). Such processing is inefficient and yields polycrystalline silicon TFTs that have relatively low field effect mobility and poor transfer characteristics. Polycrystalline silicon produced by solid-phase crystallization of as-deposited amorphous silicon on glass suffers due to small crystal size and a high density of intragrain defects in the crystalline structure.
Excimer laser annealing (ELA) has been actively investigated as an alternative to low-temperature solid-phase crystallization of amorphous silicon on glass. In excimer laser annealing, a high-energy pulsed laser directs laser radiation at selected regions of the target film, exposing the silicon to very high temperatures for short durations. Typically, each laser pulse covers only a small area (several millimeters in diameter) and the substrate or laser is stepped through an exposure pattern of overlapping exposures, as is known in the art.
Lateral crystallization by excimer laser annealing (LC-ELA) is one method that has been used to form high quality polycrystalline films having large and uniform grains. LC-ELA also provides controlled grain boundary location.
According to one method of conducting LC-ELA, an initially amorphous silicon film is irradiated by a very narrow laser beamlet, typically 3-5 micrometers wide. Passing a laser beam through a mask that has slits forms the beamlet, which is projected onto the surface of the silicon film.
The beamlet crystallizes the amorphous silicon in its vicinity forming one or more crystals. The crystals grow within the area irradiated by the beamlet. The crystals grow primarily inward from edges of the irradiated area toward the center. The distance the crystal grows, which is also referred to as the lateral growth length, is a function of the amorphous silicon film thickness and the substrate temperature. Typical lateral growth lengths for 50 nm films is approximately 1.2 micrometers. After an initial beamlet has crystallized a portion of the amorphous silicon, a second beamlet is directed at the silicon film at a location less than half the lateral growth length from the previous beamlet. Moving either the laser, along with its associated optics, or by moving the silicon substrate, typically using a stepper, changes the location of the beamlet. Stepping a small amount at a time and irradiating the silicon film causes crystal grains to grow laterally from the crystal seeds of the poly-Si material formed in the previous step. This achieves lateral pulling of the crystals in a manner similar to zone-melting-crystallization (ZMR) methods or other similar processes.
As a result of this lateral growth, the crystals produced tend to attain high quality along the direction of the advancing beamlets, also referred to as the xe2x80x9cpulling direction.xe2x80x9d However, the elongated crystal grains produced are separated by grain boundaries that run approximately parallel to the long grain axes, which are generally perpendicular to the length of the narrow beamlet.
When this poly-Si material is used to fabricate electronic devices, the total resistance to carrier transport is affected by the combination of barriers that a carrier has to cross as it travels under the influence of a given potential. Due to the additional number of grain boundaries that are crossed when the carrier travels in a direction perpendicular to the long grain axes of the poly-Si material, the carrier will experience higher resistance as compared to the carrier traveling parallel to the long grain axes. Therefore, the performance of TFTs fabricated on poly-Si films formed using LC-ELA will depend upon the orientation of the TFT channel relative to the long grain axes, which corresponds to the main growth direction. Typically, TFT performance varies by a factor of between 2 and 4 as a function of orientation relative to the main growth direction.
This difference in performance is undesirable from the point of view that as LCD resolution increases, or as panel size decreases, size limitations make it more desirable to have column drivers and row drivers oriented at ninety degrees relative to each other, potentially resulting in one set of drivers having significantly different characteristics relative to the other.
Accordingly, a method of forming polycrystalline regions on a substrate is provided. A first mask orientation is selected. A laser beam is directed through the mask to irradiate the substrate over an initial region on the substrate. The region is annealed using a lateral crystallization process. A second mask orientation is selected. The laser beam is directed through the mask to irradiate the substrate over a second region on the substrate. The region is annealed using a lateral crystallization process. If the first and second mask orientations are different, the first region will have a different crystal orientation than the second region following annealing. The mask orientation is selected by rotating the mask or the substrate.
The method of the present invention is well suited for processing LCD devices. An LCD substrate, which can be composed of quartz, glass, plastic or other suitable transparent material, is used. An amorphous semiconductor material is deposited on the LCD substrate to form a thin layer of amorphous silicon. Preferably the semiconductor material will be silicon. A first region of the amorphous silicon is annealed using a first mask orientation in connection with a lateral crystallization ELA process to form a first polycrystalline region having elongated grain structures with a first crystal orientation. A second region of the amorphous silicon is annealed using a second mask orientation in connection with a lateral crystallization ELA process to form a second polycrystalline region having elongated grain structures with a second crystal orientation. The second crystal orientation is different from the first crystal orientation, and preferably the crystal orientations are substantially perpendicular with respect to each other.
For certain applications it is desirable to form thin film transistors (TFTs) using the polycrystalline material formed by laser annealing. In a preferred embodiment of the present method, TFTs having a first channel orientation are formed over the region with the first crystal orientation. The channel orientation is preferably substantially parallel to the crystal orientation, whereby the fewest number of crystal grain boundaries are crossed by the channel. TFTs having a second channel orientation formed over the region with the second crystal orientation.